The entire disclosure of Japanese Patent Application No. 2009-022711, filed Feb. 3, 2009 is expressly incorporated by reference herein.
1. Technical Field
The present invention relates to integrated circuit devices and electronic apparatuses.
2. Related Art
In recent years, in order to achieve higher integration and lower power consumption of semiconductor integrated circuit devices, a plurality of LSIs with different power source voltages may be connected or circuits with different power supply voltages may be mounted on the same chip. For this reason, an input output circuit (I/O circuit) needs to be a tolerant I/O circuit that operates without any interference even when input signals with a voltage higher than the power supply voltage are inputted in its I/O terminals. Therefore, as one example of such a tolerant I/O circuit, JP-A-2000-77996 (Patent Document 1) describes a voltage tolerant circuit that may be used as an I/O circuit in which a potential that is different and higher than the power supply potential within the circuit would possibly be inputted from outside. In the I/O cell provided in the I/O unit of such a semiconductor integrated circuit device, an electrostatic protection element (an electrostatic protection diode and the like) is provided for protecting the semiconductor integrated circuit device against electrical stresses such as external electrostatic induction voltages and excess currents.
The inventor of the present invention conducted studies on behaviors of internal circuits of integrated circuit devices (IC) when the ground line (the low potential power supply line) becomes to be an open state (in a floating state) due to adhesion of dielectric foreign matters or the like. As a result, it has become clear that, even when the ground line becomes open, the internal circuit may operate because a current path that runs through the electrostatic protection diode is formed.
However, the current path is formed only when the potential on a terminal to be connected to the electrostatic protection diode (i.e., a terminal other than the ground terminal) is at L (=GND), and the current path is not formed when the potential on that terminal becomes to be H, such that the internal circuit stops operating.
In other words, it has been found that, when the ground line becomes open, the internal circuit operates when the potential on the other terminal is at L level, and the internal circuit does not operate when the same is at H level, in other words, non-regular circuit operations may occur. In this case, operation/non-operation of the internal circuit of the IC is determined by the potential on the terminal to be connected to the electrostatic protection diode, which makes it impossible to secure the reliability of the IC.